1. Field of the Invention
The present invention relates to the use of graphite nonwovens and graphite woven fabrics for removing cellular constituents from blood and corresponding test agents for analysis of blood constituents.
2. Description of Related Art
The separation of serum or plasma from whole blood is of overriding importance in clinical chemistry. In particular, many diagnostic detection reactions of blood components proceed without impairment only after the red blood corpuscles have been separated off. This particularly applies to colour reactions which are evaluated either by reflectometry or visually, or also electrochemically.
The most common form for separating off erythrocytes is centrifugation. However, this presents problems with small blood samples in particular, so that a whole series of aids are known for this, as described, for example, in German Auslegeschrift 25 59 242.
The use of special erythrocyte retention substrates in the field of whole blood analysis with the aid of test strips, such as blood sugar monitoring under home user conditions, is of particular importance.
As prior art, multi-layer test systems comprising a reagent layer and an erythrocyte separation zone comprising at least one or more glass fibre layers, which may be different, have proved themselves here (U.S. Pat. No. 4,477,575).
The whole blood is applied to the glass fibre layer, the erythrocytes being adsorbed into this layer as a consequence of agglutination, while plasma and serum diffuse into the reagent layer, where the detection reaction can proceed without interference by erythrocytes.
As described in EP 0 133 895, the glass fibre layer can comprise auxiliary reagents, such as certain polar dyestuffs, which have the effect of coagulation or agglutination of the erythrocytes.
According to German Offenlegungsschrift 30 29 579, the glass fibres can be loosely stacked and processed in the form of papers, nonwovens or felts, columns packed with glass fibres also being claimed. According to U.S. Pat. No 4,477,575, the glass fibres used can have a diameter in the range from 0.2 to 5.0 .mu.m and be in the density range from 0.1 to 0.5 g/cm.sup.2.
A disadvantage of the diagnostic test systems with erythrocyte retention substrates of glass fibre nonwovens is their relatively high requirement of whole blood volumes, which are about 10 .mu.l for the known Reflotron.RTM. glucose test systems. However, smaller amounts of blood, for example 5 .mu.l or less, are of great advantage, in particular in respect of obtaining the blood as painlessly as possible.
Another important disadvantage of the glass fibre nonwovens is their low mechanical strength, which is even significantly below the values such as are known, for example, for thin blotting papers.
Mechanical working, for example cutting or impregnation using conventional machines which require certain tear strengths, is made exceptionally difficult as a result.
WO 94/27140 describes erythrocyte retention layers of porous membrane matrices which comprise, for example, dextrans, polylysines, polybrenes or protamines as agglutinating agents. However, compared with the abovementioned glass fibre systems, these membrane layers are more complicated to produce and are not so variable in respect of layer thickness, absorption volume and flow or transportation properties, it being impossible, in particular, to realize horizontal, chromatography-like transportation functions, such as are required for realization of certain test strip formats (for example FIG. 4 in U.S. Pat. No. 4,477,575).
As in the case of the abovementioned white glass fibre layers, after application of blood an intense red discoloration of the retention layers occurs. Because of this red background coloration, undesirable interference problems may occur in reflectometric evaluation of the reagent layers on top.